1. Field of the Invention
The present invention concerns a frequency multiplier circuit with variable order of multiplication.
The invention can be applied in particularly, but not restrictively, to the generation of signals by frequency synthesis. This is a technique commonly used in radars and telecommunications systems.
2. Description of the Prior Art
In effect, frequency multiplication is one of the basic operations in frequency synthesis, and, to this end, it is advantageously possible to use a frequency multiplier with a variable multiplication order, either by mechanical or analog control or by digital control (we then refer to a programmable multiplication order).
There are many known frequency multiplication circuits, based on the production of energy distributed on harmonics of a basic frequency applied to the input of the multiplier.
When variable or programmable and/or high order multiplication (namely with an order of more than 5) is sought, a circuit of the type shown in FIG. 1 is generally used. This circuit essentially has a comb generator 1, associated with a tunable pass-band filter 2.
The signal with the base frequency f.sub.e, applied at input to the comb generator, is deformed by the non linear transfer characteristic of this generator into a temporal wave which ha the shape shown in A, the frequential image of this temporal wave shown in B being a comb spectrum having a large number of harmonics of the base frequency f.sub.e.
The signal A,B is applied to a tunable pass-band filter 2 which selects only the N order harmonic line (illustrated in B'), i.e. corresponding to the frequency N.f.sub.e and to it alone (leaving out surrounding unwanted lines which result from the fact that the response of the filter never as perfectly sharp edge's). The corresponding temporal wave in shown at A'.
For the comb generator 1, a snap-off type diode has generally been used (this component is well known for its microwave applications) the very sharp reaction of switching over from the forward direction to the reverse direction enabling the generation of high order harmonics with an energy level that is relatively constant over a wide range of harmonic frequencies.
Nonetheless, this assembly has certain drawbacks which are inherent to the properties of snap-off diodes, notably low efficiency, high intrinsic phase noise, and sensitivity to charge and to the environment.
To restrict these drawbacks, it is necessary to provide for high driving power at the input of the comb filter, as well as a separating stage between the comb generator 1 ad the pass-band filter 2 to prevent the impedance characteristics of the latter from disturbing the operation of the comb generator.